Lithium-ion batteries have found widespread usage as electrical energy storage devices in various portable electronics because of their light weight relative to other types of batteries. However, particularly for high power applications such as electric vehicles, there has been a continuing effort to improve the energy output and useful lifetime in lithium ion batteries to better suit these high power applications.
Lithium-sulfur (Li/S) batteries, in particular, hold great promise for high power applications. Lithium-sulfur batteries have a theoretical capacity of 1675 mAhg−1, nearly one magnitude higher than that of LiFePO4 (theoretical capacity of 176 mAhg−1). Nevertheless, the Li/S system has not yet been implemented in high power applications because of two significant obstacles: the poor electrical conductivity of elemental sulfur and the intrinsic polysulfide shuttle, both of which contribute to capacity fade with cycling.
The electrical conductivity of elemental sulfur is as low as 5×10−30 S/cm at 25° C. Such a low conductivity causes poor electrochemical contact of the sulfur and leads to low utilization of active materials in the cathode. Although compositing elemental sulfur with carbon or conducting polymers significantly improves the electrical conductivity of sulfur-containing cathodes, the porous structure of the cathode still needs optimization to facilitate the transport of ions while retaining the integrity of the cathode after dissolution of sulfur at the discharge cycle.
The sulfur in the cathode, except at the full charge state, is generally present as a solution of polysulfides in the electrolyte. The concentration of polysulfide species Sn2− with n greater than 4 at the cathode is generally higher than that at the anode, and the concentration of Sn2− with n smaller than 4 is generally higher at the anode than the cathode. The concentration gradients of the polysulfide species drive the intrinsic polysulfide shuttle between the electrodes, and this leads to poor cyclability, high current leakage, and low charge-discharge efficiency.
Most importantly, a portion of the polysulfide is transformed into lithium sulfide, which is deposited on the anode. This deposition process occurs in each charge/discharge cycle and eventually leads to the complete loss of capacity of the sulfur cathode. The deposition of lithium sulfide also leads to an increase of internal cell resistance due to the insulating nature of lithium sulfide. Progressive increases in charging voltage and decreases in discharge voltage are common phenomena in lithium-sulfur batteries because of the increase of cell resistance in consecutive cycles. Hence, the energy efficiency decreases with the increase of cycle numbers.
Much research has been conducted to mitigate the negative effect of the polysulfide shuttle. The bulk of this research has focused on either the protection of lithium anode or the restraining of the ionic mobility of the polysulfide anions. However, protection of the lithium anode leads to the passivation of the anode, and this in turn causes a slow reaction rate of the anode during the discharge cycle. Therefore, protection of the lithium anode leads to the loss of power density. Gel electrolytes and solid electrolytes have also been used as a means for slowing down the polysulfide shuttle by reducing the ionic mobility of electrolytes. However, the slow transport of ions leads to a low power density. Moreover, neither the protection of lithium anode nor the restraining of ionic mobility completely shuts down the polysulfide shuttle. Although the polysulfide shuttle occurs at slow speed, such modified Li/S batteries generally suffer from a significantly shortened lifespan as compared to lithium ion batteries without these modifications.
Accordingly, there is a need for lithium-sulfur batteries with improved performance, particularly with respect to initial discharge capacities, cycling performance, rate capability, and electrical power output (i.e., improved power density), as well as improved usable lifetime. There would be a particular benefit in a lithium-sulfur battery possessing both an improved power output and an improved usable lifetime. In achieving the aforementioned goals, there is a particular need for a lithium-sulfur battery design that minimizes or altogether prevents the irreversible deposition of lithium sulfide on the lithium anode of the battery.